1. Field of the Invention
The present invention relates to a system for controlling the charging of a secondary battery, and particularly to a system for controlling the charging of a secondary battery, which is suitable for enhancing the durability of the secondary battery, that is, keeping an initial charging/discharging performance of the secondary battery for a long-period of time.
2. Description of Background Art
In the case of charging a secondary battery using nickel oxide as the material for a cathode (positive electrode), for example, a nickel-cadmium battery or a nickel-hydrogen battery, as the battery is charged to a full-charge level and oxygen gas is generated at the positive electrode. If the generation of the oxygen gas is continued, the inner pressure of the secondary battery is raised, and accordingly, the oxygen gas thus generated is required to be consumed by local cell reaction on the surface of an anode (negative electrode). In general, taking into account the gradual reduction in capacity of the negative electrode by the local cell reaction, the capacity of the negative electrode is set to be larger than that of the positive electrode. For example, a ratio (NP ratio) of the capacity of the negative electrode to that of the positive electrode is set to a value in a range of about 1.65 to 2.0. However, since the charging capacity of the battery is increased with the positive electrode taken as a rate-limiting factor, the increased capacity of the negative electrode brings enlargement of the secondary battery but does not contribute to the increase in charging capacity of the battery.
A charging method capable of suppressing the generation of oxygen gas by stopping the charging at a charge level less than a full-charge level, for example, 97% of the full-charge level has been proposed in Japanese Patent Laid-open No. Hei 5-111175. This charging method is advantageous in that since the generation of oxygen gas is suppressed, the reduction in capacity of a negative electrode by local cell reaction can be prevented, with a result that it is possible to reduce the capacity of the negative electrode, and hence to miniaturize the secondary battery, that is, increase the charging capacity.
As described above, the stopping of the charging at a charge level less than a full-charge level is desirable from the viewpoint of durability of a secondary battery. However, since a voltage reference value (cut-off voltage) representative of a reference charge amount for stopping the charging is dependent on battery temperature and on environmental temperature and also on the degree of deterioration corresponding to the service period, it is not easy to accurately detect that the voltage of the secondary battery reaches the cut-off voltage. In other words, it is not expected to accurately control the charging of the secondary battery only on the basis of the cut-off voltage.